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Introduction to Rotational Molding 11 1.5 General Relationships between Processing Conditions and Properties The rotational molding process is unique among molding methods for plastics in that the plastic at room temperature is placed in a mold at approximately room temperature and the whole assembly is heated up to the melting tempera- ture for the plastic. Both the mold and the plastic are then cooled back to room temperature. Normally, the only controls on the process are the oven tempera- ture, the time in the oven, and the rate of cooling. Each of these variables has a major effect on the properties of the end product and this will be discussed in detail in later chapters. At this stage it is useful to be aware that if the oven time is too short, or the oven temperature is too low, then the fusing and consoli- dation of the plastic will not be complete. This results in low strength, low stiffness, and a lack of toughness in the end product. Conversely, if the plastic is overheated then degradation processes will occur in the plastic and this results in brittleness.42ñ44 In a commercial production environment the opti- mum ìcookingî time for the plastic in the oven often has to be established by trial and error.45 In recent years it has been shown that if the temperature of the air inside the mold is recorded throughout the molding cycle, then it is possibletoobserveinrealtimemanykeystagesintheprocess.46,47 Thistech- nology will be discussed in detail in Chapter 5. At this stage an overview will be given of the relationships between processing conditions and the quality of the molded part. It is important to understand that rotational molding does not rely on centrifugal forces to throw the plastic against the mold wall. The speeds of rotation are slow, and the powder undergoes a regular tumbling and mixing action. Effectively the powder lies in the bottom of the mold and different points on the surface of the mold come down into the powder pool. The regu- larity with which this happens depends on the speed ratio, that is the ratio of the major (arm) speed to the minor (plate) speed. The most common speed ratio is 4:1 because this gives a uniform coating of the inside surface of most mold shapes. The importance of the speed ratio in relation to the wall thick- ness distribution will be discussed in Chapter 5. When the mold rotates in the oven, its metal wall becomes hot, and the surface of the powder particles becomes tacky. The particles stick to the mold wall and to each other, thus building up a loose powdery mass against the mold wall. A major portion of the cycle is then taken up in sintering the loose powdery mass until it is a homogeneous melt.48ñ50 The irregular pockets ofPDF Image | INTRODUCTION TO ROTATIONAL MOLDING 1
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